Structured substrate for optical fiber alignment

ABSTRACT

A structured substrate for optical fiber alignment is produced at least in part by forming a substrate with a plurality of buried conductive features and a plurality of top level conductive features. At least one of the plurality of top level conductive features defines a bond pad. A groove is then patterned in the substrate utilizing a portion of the plurality of top level conductive features as an etch mask and one of the plurality of buried conductive features as an etch stop. At least a portion of an optical fiber is placed into the groove.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 14/981,880 filed Dec. 28, 2015 entitled “STRUCTURED SUBSTRATEFOR OPTICAL FIBER ALIGNMENT,” which is in turn a divisional of U.S.patent application Ser. No. 13/970,089 filed Aug. 19, 2013, entitled“STRUCTURED SUBSTRATE FOR OPTICAL FIBER ALIGNMENT,” now U.S. Pat. No.9,246,592. The complete disclosures of the aforementioned U.S. patentapplications Ser. Nos. 13/970,089 and 14/981,880 are expresslyincorporated herein by reference in their entireties for all purposes.

STATEMENT OF GOVERNMENT RIGHTS

Not Applicable

FIELD OF THE INVENTION

The present invention relates to the electrical, electronic and computerarts, and, more particularly, to optical fibers and optoelectronicdevices.

BACKGROUND OF THE INVENTION

The performance of computer systems continues to improve as the numberof processing cores increases. However, this increase in the number ofprocessors also requires a corresponding improvement in a system'sinterconnect bandwidth in order for the full performance advantage to berealized.

Fiber optic data links, when packaged close to a processor and/or aswitch chip, provide dramatic improvements in interconnect bandwidth andenable high speed data communications over greater distances. Fiberoptic data links are typically facilitated by the use of opticaltransceivers. Quad Small Form-factor Pluggable (QSFP or QSFP+) opticaltransceivers are, for example, frequently utilized to interfaceswitches, routers, media converters, and similar devices to opticalfibers. The QSFP+ specification supports Serial Attached SCSI (SAS),Ethernet, Fibre Channel, Infiniband, and other communicationapplications. Each of the four transceiver channels may operate at adata rate of 1 to 10.5 gigabits per second and support a reach of up to100 meters.

SUMMARY OF THE INVENTION

Aspects of the invention provide methods of forming apparatus that areadapted to couple optoelectronic devices with optical fibers, whileproviding precise alignment therebetween. Advantageously, theseembodiments may have small part counts and may be fabricated usingrelatively straightforward and low cost processing methodologies.

In accordance with an aspect of the invention, a structured substratefor optical fiber alignment is produced at least in part by forming asubstrate with a plurality of buried conductive features and a pluralityof top level conductive features. At least one of the plurality of toplevel conductive features defines a bond pad. A groove is then patternedin the substrate utilizing a portion of the plurality of top levelconductive features as an etch mask and one of the plurality of buriedconductive features as an etch stop. At least a portion of an opticalfiber is placed into the groove.

Moreover, in accordance with another aspect of the invention, anapparatus is produced at least in part by forming a substrate with aplurality of buried conductive features and a plurality of top levelconductive features. At least one of the plurality of top levelconductive features defines a bond pad. A groove is then patterned inthe substrate utilizing a portion of the plurality of top levelconductive features as an etch mask and one of the plurality of buriedconductive features as an etch stop. At least a portion of an opticalfiber is placed into the groove.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of an optical transceiver in accordancewith a first illustrative embodiment of the invention;

FIG. 2 shows a magnified perspective view of a portion of the FIG. 1optical transceiver;

FIG. 3 shows a sectional view of a portion of the FIG. 1 opticaltransceiver;

FIG. 4 shows a sectional view of an illustrative structure similar tothe FIG. 1 optical transceiver;

FIG. 5 shows a flow diagram of a method for forming the FIG. 1 opticaltransceiver, in accordance with an illustrative embodiment of theinvention;

FIG. 6 shows a sectional view of an etching process in the FIG. 5method;

FIG. 7 shows a perspective view of a portion of the FIG. 1 opticaltransceiver during formation using the FIG. 5 method;

FIG. 8 shows a perspective top view of a portion of the FIG. 1 opticaltransceiver during formation using the FIG. 5 method;

FIG. 9 shows a perspective view of an illustrative package that may beused with the FIG. 1 optical transceiver, in accordance with anillustrative embodiment of the invention;

FIG. 10 shows a perspective view of a portion of an optical fiber with amirror and lens, in accordance with an illustrative embodiment of theinvention;

FIG. 11 shows a side perspective view of the FIG. 10 optical fiberportion;

FIG. 12 shows a ray tracing diagram for the FIG. 10 lens;

FIG. 13 shows a diagrammatic representation of the underside of anoptoelectronic device, in accordance with an illustrative embodiment ofthe invention;

FIG. 14 shows a perspective view of an alternative optical transceiver,in accordance with a second illustrative embodiment of the invention;

FIG. 15 shows a magnified top view of a portion of the FIG. 14 opticaltransceiver; and

FIGS. 16-18 show various sectional views of the FIG. 15 opticaltransceiver portion.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will be described with reference to illustrativeembodiments. For this reason, numerous modifications can be made tothese embodiments and the results will still come within the scope ofthe invention. No limitations with respect to the specific embodimentsdescribed herein are intended or should be inferred.

As used herein, the terms “conductive” and “insulating” are intended tomean electrically conductive and electrically insulating, respectively,unless specifically stated otherwise.

FIG. 1 shows a perspective view of an apparatus in accordance with anillustrative embodiment of the invention. For purposes of describingaspects of the invention, the apparatus forms a portion of an opticaltransceiver 100. Nevertheless, it is emphasized that aspects of theinvention are more generally applicable to any application whereinoptical fibers terminate in a substrate and need to be precisely alignedwith one or more optoelectronic devices mounted thereon.

The non-limiting, illustrative optical transceiver 100 comprises asubstrate 105, two optoelectronic devices 110, and two integratedcircuits 115. Eight optical fibers 120 form four receive and fourtransmit channels, and, after being received by the substrate 105, arerouted through a fiber holder 125. After leaving the fiber holder 125,the optical fibers 120 ultimately terminate immediately underneath thetwo optoelectronic devices 110. At the opposite end of the substrate105, a set of exposed conductive lines form a module edge connector 130,which allows the optical transceiver 100 to interface with an externalsystem.

Additional aspects of the optical transceiver 100 are further elucidatedin FIGS. 2-4, where FIG. 2 shows a magnified perspective view of aportion of the optical transceiver 100, and FIG. 3 shows a sectionalview cut along the longitudinal axis of one of the optical fibers 120.FIG. 4, moreover, shows a sectional representation transverse to anoptical fiber of an illustrative structure similar to the opticaltransceiver 100 that highlights the relationship between the opticalfibers 120 and an overlying optoelectronic device 110 (and is thereforemarked with like reference numerals). The substrate 105 supports bothelectrical interconnections between the optoelectronic devices 110, theintegrated circuits 115, and the module end connector 130, as well asoptical interconnections between the optoelectronic devices 110 and theoptical fibers 120. For purposes of providing electricalinterconnections between the optoelectronic devices 110, the integratedcircuits 115, and the module end connector 130, the substrate 105comprises four levels of electrically conductive features: first levelconductive features 135, second level conductive features 140, thirdlevel conductive features 145, and top level conductive features 150.These conductive features 135, 140, 145, 150 are separated bothlaterally and vertically by insulating material 155 (i.e., dielectricmaterial), while also being interconnected vertically by vertical vias(not specifically shown) so as to form the desired electrical pathways.At the same time, the top level conductive features 150 are separatedlaterally from each other by a top insulating layer 160, but areotherwise exposed. Exposed regions of the top level conductive features150 form electrical traces 165, visible between the optoelectronicdevices 110 and the integrated circuits 115. Moreover, the top levelconductive features 150 form bond pads 170 required to couple thesedevices 110, 115 to the substrate 105. Lastly, additional top levelconductive features 150 also define the module edge connector 130.

For purposes of providing the required optical interconnections betweenthe optoelectronic devices 110 and the optical fibers 120, moreover, thesubstrate 105 further defines a set of eight grooves 175 and a largertrench 180. A respective end portion of each of the optical fibers 120is located in a respective one of the grooves 175, and the fiber holder125 is partially disposed within the trench 180. For reasons detailedbelow, each of the upper edges of the grooves 175 and the trench 180 areabutted by respective top level conductive features 150. Moreover, thebottoms of each of the grooves 175 and the trench 180 are formed byrespective third level conductive features 145.

Lastly, the optoelectronic devices 110 and the integrated circuits 115in the illustrative optical transceiver 100 are connected to the bondpads 170 on the substrate 105 via solder balls 185 (i.e., the devices110, 115 are flip-chipped to the substrate 105). The solder balls 185serve several functions. They mount the devices 110, 115 to thesubstrate 105; provide conductive electrical pathways between thedevices 110, 115 and the substrate 105; and dissipate heat from thedevices 110, 115 to the substrate 105. Given that the illustrativedevice is an optical transceiver, one of the optoelectronic devices 110may comprise four photodetectors (e.g., silicon photodiodes, indiumgallium arsenide photodiodes, avalanche photodiodes), while the otheroptoelectronic device 110 may comprise four light sources (e.g., lightemitting diodes (LEDs), Fabry-Perot lasers, distributed feedback lasers,vertical cavity surface-emitting lasers (VCSELs)). The opticaltransceiver 100 is therefore able to receive light signals over fourchannels and to transmit light signals over four channels.

FIG. 5 goes on to show a flow diagram of a method 500 for forming theabove-described optical transceiver 100, in accordance with anillustrative embodiment of the invention. Advantageously, the method 500utilizes a novel combination of design and processing steps to allow thegrooves 175 and the trench 180 to be self-aligned to the bond pads 170.This self-alignment, in turn, helps to assure that the ends of theoptical fibers 120 are ultimately positioned correctly in relation tothe optoelectronic devices 110 so as to create efficient opticalcommunications therebetween. At the same time, this passive alignment isaccomplished by a relatively straightforward and low cost processingscheme, while also creating an optical transceiver 100 with a relativelysmall part count.

Step 505 of the method 500 comprises forming the substrate 105, which,in the present illustrative embodiment, may comprise a surface laminarcircuit (“SLC”; also sometimes called a “sequential build-up” (SBU)circuit board). SLCs are presently widely used in telecommunication andvideo camcorder products, and thus their formation will already befamiliar to one having ordinary skill in the packaging arts. Moreover, arepresentative method of forming SLCs is taught in U.S. Pat. No.5,451,721 to Tsuchida et al., entitled “Multilayer Printed Circuit Boardand Method for Fabricating Same,” which is hereby incorporated byreference herein. In the present embodiment, the insulating material 155and the top insulating layer 160 may, as just one example, comprise anepoxy, while the conductive features 135, 140, 145, 150 may comprise, asjust another example, a metal such as copper.

The conductive features 135, 140, 145, 150 in the substrate 105 serve atleast five purposes: 1) defining the bond pads 170 that facilitate theconnections of the optoelectronic devices 110 and the integratedcircuits 115 to the substrate 105; 2) defining the module end connector130; 3) defining the electrical traces 165 (both internal and external)and vertical vias that route electrical signals between theoptoelectronic devices 110, the integrated circuits 115, and the moduleend connector 130; 4) acting as a mask for the formation of the grooves175 and the trench 180 associated with the placement of the opticalfibers 120; and 5) acting as etch stops for these same grooves 175 andtrench 180. Accordingly, these various functions are preferablyconsidered when laying out the several layers of conductive features135, 140, 145, 150. To utilize a portion of the top level conductivefeatures 150 as an etch mask for the grooves 175 and the trench 180, forexample, the top level conductive features 150 are preferably patternedin those regions of the substrate 105 where the grooves 175 and thetrench 180 are to be placed such that the top level conductive features150 occupy the edges of the intended grooves 175 and trench 180. Thatis, the top level conductive features 150 are patterned in the shape ofa developed photoresist mask in relation to the intended grooves 175 andtrench 180. At the same time, to further utilize the conductive features135, 140, 145, 150 as an etch stop for the grooves 175 and the trench180, buried conductive features (in this particular embodiment, thirdlevel conductive features 145) are preferably placed in the same regionsof the substrate 105 where the grooves 175 and the trench 180 areintended so that the buried conductive features will fall at the bottomsof the grooves 175 and the trench 180.

With the conductive features 135, 140, 145, 150 so patterned, formingthe grooves 175 and the trench 180 simply becomes a process of etchingthe regions of the substrate 105 where the grooves 175 and the trench180 are intended utilizing the top level conductive features 150 as anetch mask and the third level conductive features 145 as an etch stop.Such a process is set forth in step 510 of the method 500, and isillustrated in a sectional view in FIG. 6. Etching may be accomplishedby, for example, laser ablation using, as just one example, a carbondioxide (CO₂) laser (labeled in FIG. 6 as laser 600). Because of theirinherent reflectivity, metal features are particularly good masks andetch stops for many laser ablation processes. Advantageously, the use ofthe top level conductive features 150 in this manner self-aligns thegrooves 175 and the trench 180 to the bond pads 170, which are likewisedefined by the top level conductive features 150. The use of an etchstop also allows the grooves 175 and the trench 180 to be fabricatedwith much greater depth uniformity than would be available in, forexample, a timed etching process.

After forming the grooves 175 and the trench 180, the substrate 105appears as shown in perspective view in FIG. 7. In step 515, the ends ofoptical fibers 120 are placed into the grooves 175 while the fiberholder 125 is partially placed into the trench 180. The ends of theoptical fibers 120 are thereby precisely placed in relation to the bondpads 185 that will support the optoelectronic devices 110. FIG. 8 showsa top view of a portion of the optical transceiver 100 at this point inthe processing, which diagrammatically shows this relationship. In step520, the optoelectronic devices 110 and the integrated circuits 115 areflip chipped to the substrate 105, resulting in the optical transceiver100 shown in FIGS. 1-3. The process of flip chipping will already befamiliar to one skilled in the packaging arts. Flip chip attachment isalso described in, for example, H-M Tong et al., Advanced Flip ChipPackaging, Springer, 2012, which is further hereby incorporated byreference herein.

Lastly, with the optical transceiver 100 formed as described above, itmay be further packaged, as indicated in step 525. FIG. 9 shows aperspective view of an illustrative package 900 that may be used withthe optical transceiver 100. This illustrative package 900 defines aQSFP+ Power Active Optical Cable connector in accordance with theSFF-8436 Specification (promulgated by the SFF Committee).

If so desired, the optical transceiver 100 lends itself to the placementof a backside monitor photodetector. This option is diagrammaticallyillustrated in FIG. 3. Here, an empty via 300 is formed immediatelyunderneath the terminus of the optical fiber 120. A backside photodiode305 is then placed so that it can detect light emitted from the opticalfiber 120 through the empty via 300.

Moreover, as another option, one or more of the optical fibers 120 inthe illustrative optical transceiver 100 may be cleaved to form a totalinternal reflection (TIR) mirror and/or a lens that may aid in directinglight signals between the optical fiber 120 and its correspondingoverlying optoelectronic device 110. FIGS. 10 and 11 show an obliqueperspective view and a perspective side view, respectively, of anoptical fiber 120 with both a TIR mirror 1000 and a lens 1005 formedthereon. Both features 1000, 1005 may be formed by conventional lasercleaving. The TIR mirror 1000 is preferably angled so as to reflect thelight at an angle of about 90 degrees. The lens 1005, in turn focusesthe light onto a plane some distance from the lens 1005. FIG. 12 shows aray tracing diagram of light at various positions in the core of theoptical fiber 120 being effectively focused onto a plane. In one or moreembodiments, a photodiode may be positioned at this plane to receivelight from the optical fiber 120.

While the illustrative optical transceiver 100 services four transmitand four receive channels via eight optical fibers 120, alternativeembodiments falling within the scope of the invention may also servicefewer or greater numbers of optical fibers. FIG. 13, for example, showsa diagrammatic representation of the underside of an optoelectronicdevice 1300 operative to service twelve optical fibers. In thisillustrative, non-limiting embodiment, one row of alternating activeregions 1305 and bond pads 1310 run along one lengthwise edge of theoptoelectronic device 1300. In use, this row would overlie the terminiof the optical fibers disposed in their respective grooves. A row ofbond pads 1315 run along the other lengthwise edge. The active regions1305 may comprise, for example, photodetectors (e.g., photodiodes) orlight sources (e.g., LEDs, lasers).

In the optical transceiver 100, the optoelectronic devices 110 overliethe ends of the optical fibers 120. In alternative embodiments fallingwithin the scope of the invention, however, the optical fibers mayterminate short of the optoelectronic devices and the transmission oflight signals therebetween may be facilitated by intervening opticalwaveguides.

FIG. 14 shows a perspective view of aspects of an alternative opticaltransceiver 1400 that is designed in this manner, in accordance with asecond illustrative embodiment of the invention. The non-limiting,illustrative optical transceiver 1400 comprises a substrate 1405 (e.g.,an SLC), two optoelectronic devices 1410 (e.g., a photo detector and alight source), two integrated circuits 1415, and a module end connector1420. Eight optical fibers 1425 form four receive and four transmitchannels, and, in a manner similar to the optical transceiver 100,occupy respective grooves 1430 in the substrate 1405. The optical fibers1425 terminate so as to abut a set of waveguides 1435 that route lightsignals between the optical fibers 1425 and the optoelectronic devices1410.

Additional aspects of the optical transceiver 1400 are shown in FIGS.15-18, where FIG. 15 shows a magnified top view of a portion of theoptical transceiver 1400 where one of the optical fibers 1425 meets oneof the waveguides 1435, and FIGS. 16-18 show various sectional views cutalong the planes indicated in FIG. 15. In addition to the optical fiber1425 and the waveguide 1435, the portion of the optical transceiver 1400in the figures further comprises top level conductive features 1440,buried conductive features 1445, insulating material 1450, a topinsulating layer 1455, and optical adhesive 1460. The optical adhesive1460 couples the end of the optical fiber 1425 to the waveguide 1435.

In one or more embodiments, the optical transceiver 1400 may be formedusing a method similar to the method 500, except with the additionalstep of forming the waveguides 1435. As was the case for the opticaltransceiver 100, the grooves 1430 for the optical fibers 1425 arepreferably patterned by laser ablation utilizing top level conductivefeatures 1440 as a mask and buried conductive features 1445 as an etchstop (see e.g. FIG. 18). The waveguides 1435, on the other hand, may beformed by conventional deposition (e.g., spin coating) in combinationwith photolithography/etching or direct laser patterning techniques thatwill already be familiar to one skilled in the art. The waveguides 1435may comprise any one of several optical polymers, which areconventionally formed by combining a variety of monomers into differentconfigurations. Once formed in this manner, the grooves 1430 act toprecisely align the optical fibers 1425 with the waveguides 1435 so asto provide efficient optical communications therebetween.

It should again be emphasized that the above-described embodiments ofthe invention are intended to be illustrative only. Other embodimentscan use different types and arrangements of elements for implementingthe described functionality, as well as different method steps forforming these elements. These numerous alternative embodiments withinthe scope of the appended claims will be apparent to one skilled in theart.

Moreover, methods in accordance with aspects of the invention may beutilized in the fabrication of packaged devices such as, but not limitedto, optical transceivers that act to convert light signals intoelectrical signals, and vice versa. These packaged devices may beutilized in apparatus such as computer systems to facilitate fast datacommunications between two or more data processors. Such apparatus(e.g., computer systems) will also fall within the scope of thisinvention.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification and claims, specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiments were chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

What is claimed is:
 1. An apparatus formed at least in part by the stepsof: forming a substrate comprising a plurality of buried conductivefeatures and a plurality of top level conductive features, at least oneof the plurality of top level conductive features defining a bond pad;patterning a groove in the substrate utilizing a portion of theplurality of top level conductive features as an etch mask and one ofthe plurality of buried conductive features as an etch stop; and placingat least a portion of an optical fiber into the groove to form anarrangement wherein at least a portion of the optical fiber sits in thegroove adjacent to the portion of the plurality of the top levelconductive features used as an etch mask, and rests on the one of theplurality of buried conductive features used as an etch stop.